The micromeres of the 16-cell stage sea urchin embryo are strictly determined. Their developmental fate is to differentiate to primary mesenchyme which then produces the skeletal system of the embryo. It is unknown if the determination and early differentiation of these cells is the result of the localization of micromere specific maternal mRNA sequences, the transcription of new cell specific mRNAs, or a combination of both factors. cDNA clones complementary to primary mesenchyme polysomal mRNA sequences will be constructed and micromere specific sequences will be selected by differential colony hybridization. Once micromere specific mRNA sequences are identified, it will be established when they are localized by in situ hybridization experiments. The loading of these sequences onto polysomes will also be followed by reacting the cloned cDNAs with polysomal and cytoplasmic RNAs from embryos for selected stages of development from 5-24 hours post fertilization. Appearance of new primary mesenchyme specific sequences will also be monitored in developing embryos.